Collaborative Research: Developing a Methodology for Imaging Stress Transients at Seismogenic Depth: Data Analysis and Interpretation

[00:45 . 71]断层系统的时变应力状态可能是控制地震事件顺序和成核的最重要的性质。作为Earthscope的一部分，地面大地测量仪器的部署将通过对时变地表应变场的全面观测，为我们提供对应力场的重要约束。作为一种增强这些约束的方法，pi目前正在开发一种基于地震速度的应力依赖性的有源方法来测量地下应力瞬变。几十年来的大量实验室研究表明，地壳岩石的弹性特性（地震速度、衰减、各向异性）明显表现出应力依赖性。这种依赖性归因于由于裂纹表面正应力的变化而导致的微裂纹的开/闭。因此，应力变化原则上可以通过利用发震地壳弹性特性的应力敏感性来检测。几十年来，人们一直在努力利用这种压力依赖性，尽管这个目标到目前为止还难以实现。造成这种情况的主要原因有两个：1)缺乏检测应力微小变化所需的足够的延时精度；2)难以在应力和介质的地震特性之间建立可靠的校准。这两个问题是耦合的，因为校准的最佳来源是固体潮汐和大气压，两者都产生102-103 Pa数量级的弱应力扰动。探测这些源需要在实验室实验的基础上测量10-5-10-6量级的分数速度变化。pi已经在不同的尺度上进行了一系列的井间有源实验：在劳伦斯伯克利国家实验室（LBNL）设施进行了3米的间隔实验，在LBNL里士满野外站（RFS）进行了30米的间隔实验，在2公里深度上进行了300米的间隔实验，从Parkfield的SAFOD先导井射向SAFOD主井的传感器。到目前为止，他们已经完成了第一个地点的工作，在RFS进行了几次测量，并正在完成RFS的最后测试。对两个测试数据集的初步分析表明，延迟时间精度可以达到10-6阶，并且可以观测到气压和潮汐引起的走时变化。利用RFS和Parkfield的最终数据集，pi计划进行以下分析并进行数值模拟：(1)估计P波及其尾波的延迟时间；(2) S波及其尾波的延迟时间估计；(3)纵波和横波振幅测量；(4) s波分裂测量；(5)纵波和横波尾波散射场成像。数值模拟包括：1)确定相应孔隙弹性介质的特征及其对已知应力的响应；2)计算相应孔隙弹性介质的地震特性。然后，最有希望的特性将用于开发定量应力校准，通过选择适当的孔隙弹性介质来建立，该介质考虑了观察到的应力敏感性和地震特性（包括散射）。

0453471NiuThe time-varying stress state of fault systems is perhaps the single most important property controlling the sequencing and nucleation of seismic events. The deployment of surface geodetic instrumentation, as part of Earthscope, will provide us with important constraints on this stress field, through comprehensive observations of the time-varying surface strain field. As a way of augmenting these constraints, the PIs are presently developing an active-source methodology based on the stress dependence of seismic velocity to measure subsurface stress transients. Numerous laboratory studies over several decades have shown that the elastic properties (seismic velocity, attenuation, anisotropy) of crustal rocks clearly exhibit stress dependence. Such dependence is attributed to the opening/closing of microcracks due to changes in the stress normal to the crack surface. Thus stress changes can, in principle, be detected by exploiting the stress sensitivity of the elastic properties of the seismogenic crust. For decades there have been efforts to exploit this stress dependence, although this goal has thus far been elusive. There are two primary reasons for this: 1) lack of sufficient time-delay precision necessary to detect small changes in stress, and 2) the difficulty in establishing a reliable calibration between stress and the seismic properties of the medium. These two problems are coupled because the best sources of calibration are the solid-earth tides and barometric pressure, both of which produce weak stress perturbations of order 102-103 Pa. Detecting these sources requires measurement of fractional velocity changes on the order of 10-5-10-6, based on laboratory experiments. The PIs have been conducting a series of cross-hole active-source experiments at different scales: 3 m spacing at the Lawrence Berkeley National Laboratory (LBNL) facility, 30 m spacing at the LBNL Richmond Field Station (RFS), and, 300 m spacing at 2 km depth, shooting from SAFOD Pilot Hole at Parkfield to sensors at the SAFOD main hole. Thus far they have completed work at the first site, have made several measurements at the RFS, and are completing the final test at RFS. Preliminary analyses of the two test datasets suggest that it is possible to reach the required delay time precision of order 10-6, and that barometric pressure and tidally-induced changes in travel time can be observed. With the final datasets from RFS and Parkfield, the PIs are planning to conduct the following analysis together with numerical modeling: (1) delay time estimates of the P wave and its coda; (2) delay time estimates of the S wave and its coda; (3) amplitude measurements for both P and S wave; (4) S-wave splitting measurements; (5) scattered-field imaging using P- and S-wave coda. Numerical modeling includes: 1) determining the characteristics of the corresponding poroelastic medium and its response to known stresses, and 2) calculating the corresponding seismic properties of such a medium. The most promising properties are then to be used to develop a quantitative stress calibration, established through the choice of an appropriate poroelastic medium that accounts for both the observed stress sensitivity and seismic properties (including scattering).